Field of the Invention
The invention relates to a method for attenuating magnetic interference in a magnetically shielded room.
For high-sensitivity biomagnetic measurements, the test room must be shielded against external interference. As a starting point, it must be understood that the resolution of a modern Superconducting Quantum Interference Device (SQUID) magnetometer is in the order of 1 fT/Hz, while typical noise level of a laboratory room is 106 fT at 1 Hz and almost 108 fT at 0.01 Hz. Although gradiometers are conventionally used in measurements, it is necessary to attenuate the level of external interference fields. Generally, attenuation is accomplished with the help of two or a greater number of shields made from mu-metal. Also active means have been employed to improve the shield. Herein, the external magnetic field is measured outside the test room with the help of a flux-gate magnetometer, for instance, and then coils placed outside the test room have been used to improve the attenuation efficiency of the test room. Also a method has been described, wherein a Helmholtz coil placed outside the test room is used as both a magnetometer and a means for compensating for the detected field.
It is an object of the present invention to overcome the drawbacks of the abovedescribed techniques and to provide an entirely novel type of method for attenuating magnetic interference imposed on a magnetically shielded room.
The goal of the invention is achieved by way of arranging about the wall elements of a test room made from a ferromagnetic material a set of measurement coils by means of which the field causing interference can be determined. After the field of interference is known by its components, a compensating field of opposed magnitude can be established by means of compensating coils that adapted outside the test room.
The invention offers significant benefits.
The coil arrangement according to the invention makes it possible to measure the field of interference to a very high accuracy by virtue of making the shielded test room itself to act as the magnetometer. Then, the actual compensation may be carried out with the help of coils placed exterior to the test room.
A preferred embodiment of the invention also allows the field pattern to be resolved.
When necessary, even strong interference fields can be attenuated by feedback signals applied to multiturn compensating coils.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.